The year in Sweden sounds like a marvelous experience for your daughter, and it could even have a positive impact on her college outcomes, despite the pitfalls.
If she attends a Swedish school (and not an American or international school where the primary language of instruction is in English), admission officials will give her wiggle-room for lower grades … not only while she’s in Sweden but also, to some degree, when she returns. However, if you feel that she won’t be learning key concepts in major subjects like science and math while she’s away, you might consider enrolling her in online classes, either concurrent to her Swedish classes or in the summer. This should ease her transition back to a U.S. high school.
You also might want to help your daughter develop “extracurricular activities” that she can begin now and continue in Sweden. Her year away may take her off of the track to leadership positions in school organizations, but the most selective colleges see more Key Club presidents and National Honor Society secretaries than they know what to do with anyway. So perhaps your daughter can hone in on outside interests that she can do independently, both here and abroad, such as poetry-writing, photography, jewelry-making, bird-watching, etc. In fact, colleges could be especially interested if these endeavors were to take a Scandinavian twist … i.e., if the poems, photos, jewelry, etc. reflect what your daughter has learned or observed in Sweden.
When it comes time for your daughter to apply to colleges, she will probably be white, middle-class and hookless, regardless of what you decide to do about the overseas option. But having that year abroad will help your daughter to stand out in a crowd, at least a tiny bit. It will also be a horizon-expanding and possibly character-building experience that she wouldn’t otherwise get and which should trump any possible concerns you might have about the bumps it could create on the road to college.